7.8 x 10 26atoms Ne = L Ne ?

First calculate mass of sodium hydrogen phosphate (Na₂HPO₄) needed for solution:
n(Na₂HPO₄) = V(Na₂HPO₄) · c(Na₂HPO₄)
n(Na₂HPO₄) = 0,3 dm³ · 0,2 mol/dm³
n(Na₂HPO₄) = 0,06 mol
m(Na₂HPO₄) = 0,06 mol · 142 g/mol
m(Na₂HPO₄) = 8,52 g.
Measure that mass on scale using spoon and beaker. Than dissolve salt in beaker with distilled water and pour that solution in volumetric flask (size 300 mL), pour distilled water to graduation mark.